Where can I find the latest STM2 and Acknowledgement for Overstay Forms on the Thai Immigration Site?

The on line ones are out of date, I had same issue you need to get current one from immig

As John said, the form you are looking for might not exist anywhere except for the immigration office you use. Some offices have their own forms that don't match what you can find online. And even if an immigration office uses the one you find online, they might still reject it because you printed it on the wrong size paper, or printed on one side when they want 2-sided, or you printed 2-sided when they wanted 1-sided. A lot of times it's better to just get the forms at the office.

Every immigration office can use their own forms which might be slightly different

Why didn't you get the new forms while you were at the Immigration office?